Not sure why you are singling out English speakers when every monolingual would be "guilty" of this. Your comment just comes off as an attempt at using language to get some unnecessary slight at specifically English speakers

If most people aren't meeting the criteria for "fluency" under the definition you have given to it, what use is that terminology? Surely it would be more logical to adjust your criteria for "fluency" than to have it be a mostly useless term.

That's why in sociolinguisutics fluency has much more to do with whether or not all of your needs are able to be met and accomplished with the linguistic repertoire you possess. It actually is able to function as a useful term through that lens.

You've hit the nail on the head here. This is more or less the exact criteria and understanding we have of "fluency" in sociolinguistics and it is disappointing that so many others in this thread are trying so hard to come up with some neat little box they can package "language" and "fluency" together in. It reeks of 60's understandings of language and psychology (thanks Chomsky).

I'm especially happy that you brought up the idea that even native speakers are still growing in their own language. People don't realize that despite being fluent, they still have huge gaping holes in their knowledge of "their own" language. Show any common person a "lawyer-ese" document and their eyes will glaze over.

Yeah, absolutely it does. A huge part of additional language teaching is learning exactly how those differences manifest and what teachers can do to best navigate those challenges and differences. But it is undoubtedly, 100 percent possible for someone to learn a language well past the "critical period".

Man is it painful to be a sociolinguist and read this thread. People really like to throw the critical period hypothesis around without really thinking about how absurd it would be if it was definitively true that someone just can't learn a language after a certain age.